(1) Field of the Invention
The present invention relates to an image displaying device having an image cache memory.
(2) Description of the Related Art
An approach of keeping an image that is once displayed or a zoomed-down image (thumbnail image) in memory as cache data for reducing a time for waiting for an image to be displayed has been known.
Japanese Patent Laid-Open Application No. 2004-152172 describes an information processing device provided with RAM (Random Access Memory) or card memory as image cache memory for saving cache data. The information processing device uses, as identifier for associating original image data with cache data, information indicating a location for storing an image such as an image file name or information on date and time of creation or date and time of update. If the cache data of an image data to be displayed is in image cache memory, it executes image display using the cache data. If the cache data of an image data to be displayed is not in the image cache memory, it executes display using the original image data.
Generally, image cache memory takes up a smaller space than that of main memory in which the original image data is stored and is limited to the amount of cache data to store. Therefore, it is preferable to avoid storing unwanted cache data in the image cache memory so that desired cache data can be effectively stored in image cache memory.
In the information processing device described in Japanese Patent Laid-Open 2004-152172, information that indicates a location storing an image such as an image file name or information on date and time of creation or date and time of update is employed as an identifier for associating original image data with cache data. In such a manner, if the same original image data is stored as different file names, cache data will be saved for each of original image data stored with different file names. It is not preferred that a plurality of cache data are saved for the same original image data from the viewpoint of effectively storing cache data.
Recently, an image displaying system in which a plurality of image transmitting devices are connected with an image displaying device via a network is known. As this type of image displaying system, a presentation system is known, for example. In the presentation system, the image transmitting device transmits a real-time image that is played by the presentation application software (for example, the program that captured a display image) on the image displaying device and the image displaying device displays the real-time image received from the image transmitting device. In such a case, the real-time image is often given a name indicating a storage (playing) location for convenience. If the same real-time image is transmitted from different image transmitting devices to the image displaying device, the image displaying device generates cache data for each of real-time images received from the image transmitting devices. Therefore, the image displaying device cannot store cache data effectively.
A method, in which the entire image data is used to a hash value of a fixed length based on a hash function as an identifier is inherent in an image data. If an identifier for cache data is generated by this method, the determination of whether each piece of generated cache data is created from the same original image data or not can be performed based on the identifier. In such a case, however, the entire original image data is used to create the identifier for cache data. Accordingly, when the image size of the original image data is large, much time is spent reading the original image data. In addition, since the amount of calculation of the hash values increases, calculation requires a lot of time.